This invention relates generally to a method and apparatus for serial dot printing and more particularly to a method and apparatus for serial dot printing producing printing indicia of high quality. Each alphanumeric character and symbol is formed by a combination of dots. It is well known that a printer of this type can form various characters and symbols by selecting suitable dot patterns in, for example, a 5.times.7 or 7.times.9 dot matrix. A 24.times.24 dot matrix may be used for Chinese characters which require many combinations of dots. Such a 24.times.24 matrix is conventionally formed by arranging twenty-four wires in a linear row, for example, vertically and moving a paper sheet orthogonally thereto for the purpose of printing on the paper sheet.
The use of such a dot matrix printer to form characters and symbols is widespread due to the fact that it is possible to form a wide selection of alphanumeric characters and symbols while using only a limited number of wires. It is not necessary to prepare fixed fonts. However, the readability of the alphanumeric characters and symbols formed by dot printing is not of excellent quality when compared with those characters printed by means of fonts. For this reason, such a dot printer has not been considered suitable in preparing official dcuments and legal documents.
An approach for the improvement in appearance of the alphanumeric characters and symbols formed by a dot matrix printer is disclosed in U.S. Pat. No. 3,900,094. In this patent, adjacent dots are overlapped to approximate a solid line by two rows of wires arranged in a staggered pattern. In U.S. Pat. No. 4,159,882, curved or wavy portions of a printed character or symbol are smoothed by a plurality of lateral traverses of the print head combined with incremental vertical motion of the print head such that different portions of the character or symbol are printed in each traverse until the character is entirely formed. A modification of the latter approach is disclosed in U.S. Pat. No. 4,242,003 wherein different data is printed on each of two lateral passes of the print head per line of print produced. The print head and recording medium are moved relative to each other in the vertical direction between each lateral traverse.
In printing methods of the prior art, dots are printed in half space positions, which smooths the curved portions and oblique slanted lines of a character. However, as illustrated by U.S. Pat. No. 4,159,882, the head must move across the paper several times and the pins or wires in the head which are driven during each traverse are different. Therefore, different driving signals for driving the plurality of pins or wires in each traverse must be provided. The changed driving signals in each traverse are in response to segregated data on the dots to be printed for forming the character in each traverse. Therefore, the control circuitry to provide the unique data for each traverse becomes sophisticated and complicated.
What is needed is a method and apparatus for serial dot printing which provides characters of very high quality using a simple apparatus with an uncomplicated control and data system.